The present invention relates to an apparatus for controlling the spring constant of a laminated leaf spring assembly including a main spring unit and a helper spring unit.
A laminated leaf spring assembly including a main spring unit and a helper spring unit has been widely used for suspending a body, for instance the body of a vehicle. The main spring unit and helper spring unit are each provided with at least one leaf spring. The main spring unit and helper spring unit are coupled together at their center by a center bolt. The relation between the main and helper spring units is determined by the load acting on the body of a vehicle to be supported, i.e., the load acting on the laminated leaf spring assembly. When the load is low, the ends of the helper spring unit are separated from the main spring unit. As the load is gradually increased, the main spring unit gradually flexes. Until the load P becomes P.sub.1, for instance, as shown in FIG. 1, the deflection .delta. of the laminated leaf spring assembly is inversely proportional to the spring constant K.sub.1 of the main spring unit and is given as (where P&lt;P.sub.1) EQU .delta.=P/K.sub.1
When the load P becomes P.sub.1, the main spring unit comes into initial contact with the helper spring unit. As the load increases beyond P.sub.1, the area of contact is gradually increased. When the load reaches P.sub.2, the main spring unit is in complete contact with the helper spring unit. At this time, the elasticity of the helper spring acts upon the main spring unit, so that the spring constant of the laminated leaf spring assembly is K.sub.2, that is, with the load P.sub.2 the deflection .delta..sub.2 is .delta..sub.2 =P.sub.2 /K.sub.2. As the load increases from P.sub.1 to P.sub.2, the area of contact between the main spring unit and helper spring unit is gradually increased, and at the same time the spring constant of the laminated leaf spring assembly is gradually increased in a linear fashion from K.sub.1 to K.sub.2. That is, while the load is being increased from P.sub.1 to P.sub.2, the laminated leaf spring assembly acts as a progressive spring.
In the conventional laminated leaf spring assembly of this kind, the spring characteristics are determined absolutely once the form of the main spring unit and helper spring unit is determined. Thus, the deflection of the laminated leaf spring assembly is determined by the value of the load acting on the body of a vehicle. Therefore, the attitude of the body of a vehicle is liable to be influenced by the value of the load. Also, the spring constant cannot be changed even if the state of the road is changed from good to bad or vice versa. For example, when the vehicle falls into a hole, an upward load acts upon the laminated leaf spring assembly through tires or the like. When the upward load is created in this way, the laminated leaf spring assembly usually deflects in the direction opposite to the direction of deflection. Therefore, the amplitude of vibrations of the laminated leaf spring assembly in the vertical directions is increased so that its durability is reduced.